Why does this bother me so much...?

A girl I know who is one of my best friends and we have kinda been on and off with liking each other but never really went anywhere because maybe we were both scared of ruining our friendship and i might possibly be in love with her..anyway..last night we went out drinking and she was kinda having a fight with her bf over the phone...she came back and said well he said its done and turned off her phone and started drinking...seemed relatively happy..and we started playin those "have you ever,,,,?" games and someone asked "have you ever had ?" and many of us took a drink and she was just sittin there lookin at me and hesitating and her friend said "well c'mon drink up"...and she did..and i was shocked cuz i did not know about this..i figured she didn't tell me cuz I am one of her guy friends and maybe she isn't supposed to tell me..but like its weird...her bf and her were fightin for the longest time for a couple months and she still went for him? why?? I was pretty pissed off for the rest of the night but didn't show it...this is bothering me so much..and i just wanna go up to he bf/ possibly ex...and just hurt him..am i jealous? why? i mean i already decided that we might just be really good friends forever and i have dated other people but this is actually just making me rage...why am i getting so frustrated over this? its her life..she can do whatever she wants..i don't even know..maybe i just expected more from her...